Uranium metal is conventionally produced by reducing uranium tetraflouride (UF.sub.4) with magnesium, often with magnesium fluoride slag liners. This reduction of the uranium provides a product commonly referred to as a regulus or derby of uranium metal which has a crust of magnesium fluoride adhering to the derby surface. The derbies from the reduction operation are remelted into ingots by induction casting. Prior to the induction casting of a derby the crust or slag including the magnesium fluoride was supposedly removed from the derby surface by mechanical means such as chipping, thermal shocking, grit blasting, and the like.
The casting operation was previously achieved by melting the derby in a graphite crucible. A reaction occurs between the molten uranium and the graphite that introduces considerable carbon in the ingot. Therefor, since in many instances carbon concentrations greater than about 100 ppm were found to be undesirable, the graphite crucibles used in the casting operation were provided with a surface coating of yttria or zirconia which are essentially non-wettable by molten uranium. With such an yttria or zirconia coating uranium derbies which have a carbon content of about 25 ppm could be cast into ingots with the carbon content increasing to only about 75 ppm carbon which is well within the 100 ppm limitation.
During the course of analyzing ingots from casting procedures utilizing yttria or zirconia coated graphite crucibles, it was discovered that several ingots contained carbon in concentrations considerably greater than 100 ppm. It was determined that residual magnesium fluoride on the surface of the uranium derbies reacted with the coatings on the graphite crucible resulting in a breakdown of the coating so as to expose the underlying graphite to molten uranium. This exposure of the graphite resulted in reactions with the molten uranium so as to contaminate the uranium with carbon concentrations greater than 100 ppm. A suitable solution or technique was needed for removing the magnesium fluoride from the uranium derbies prior to the induction casting of the ingots to inhibit the breakdown of the crucible coatings. Several techniques previously used for removing the magnesium fluoride from the uranium derbies suffered shortcomings or drawbacks. For example, the forming of an oxide layer on the uranium surface by heating the derbies in air and then water quenching the derbies so as to remove both the oxide and the magnesium fluoride layers by thermal shocking was found to result in excess oxidation of the uranium. As pointed out above, other techniques for cleaning the surface of the uranium derbies such as chipping and grit blasting left sufficient residual magnesium fluoride on the derby to react with the crucible coatings. Also, the use of a warm nitric acid bath did not remove the residual magnesium fluoride from the surface of the uranium derbies.